Additional Info:

I did create a separate user for this. And after it worked I deleted this test user.
Then I realised that the cyrus-imap mail box was still there. And when I recreated
the user I was able to login and access the mail with all the test mails still there.
So don't forget to delete the mailbox by hand when you delete a user

Aliasing the user should work. You will still have a mailbox but mail won't be delivered. Adding users manually outside LDAP is full of dangers.

Found the answer here

Thanks for the answer. This would work, I know.

But that is not what i meant.
I have created a user in Webconfig and I want that user not to sent and
receive emails. Stop sending is no problem. I just disable IMAP and SMTP.
But the user would still get emails. Filling up a unused mailbox

Gabriel Flepp wrote:

Is it possible to create a User without an email address?
I mean I can disable the access to the IMAP server. But I can't delete the email address.
So email is still delivered and can't be read...

Try

useradd -u 1100 -g 63000 -s /bin/bash username

Check if the userid 1100 is still free
Groupid 63000 is "Allusers"